Qualitative Clar Aromatic Sextet Theory

Armit and Robinson introduced the notion of six-member rings in unsaturated compounds associated with the k electron aromatic sextet that bestows on them unusual stability, for which they have introduced the circle notation. Robinson later abandoned this novel model to characterize benzenoid compounds. It was Clar who not only adopted the notion of aromatic sextets, but significantly developed the model into a theory, which could explain several regularities of selected properties of these compounds. Clar s aromatic sextet theory assigns to individual benzenoid hydrocarbons novel structural formulas, which are obtained by following the rules summarized in Table 11.1. 

The Clar sextet theory has received fair attention in the literature and continues to remain a subject of study as is reflected in several recent publications [11-16]. Unfortunately, Clar s sextets have not yet attracted the visible attention of quantum chemists, who could investigate if the roots of Clar s theory have some hidden relationship with quantum mechanics. As the relationship of Clar s sextet theory to quantum mechanics is unexplained and unknown, we are tempted to pose this 

Rules for Construction of Clar s Formulas for Benzenoid Hydrocarbons 

FIGURE 11.1 Clar structures of smaller benzenoid hydrocarbons. 

Using the standard quantum chemical approaches and general methodology of quantum mechanics, seek non-empirical justifications of Clar s sextet theory. 

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In practice, the valence bond picture has probably exerted more influence on how chemists actually think than the HMO picture. However most early applications were primarily qualitative in nature. This qualitative VB picture can be summarized under die name of resonance theory [10]. The basic concept is that in general the more ways one has of arranging the spin pairing in the VB wave function, the more stable the molecule is likely to be. Thus, VB theory predicts that phenanthrene with 14 carbon atoms and 5 Kekule structures should be more stable than anthracene with 14 carbon atoms but just 4 Kekule structures, in complete accord with the experimental evidence. It also predicts that benzenoid hydrocarbons with no Kekule structures should be unstable and highly reactive, and in fact no such compounds are knowa Extensions of this qualitative picture appear, for example, in Clar s ideas of resonant sextets [11], which seem to be very powerful in rationalizing much of the chemistry of benzenoid aromatic hydrocarbons. The early ascendancy of HMO theory was thus largely based on the ease with which it could be used for quantitative computations rather than on any inherent superiority of its fundamental assumptions. 

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